staging: erofs: add a TODO and update MAINTAINERS for staging

This patch adds a TODO to list the things to be done, and
the relevant info to MAINTAINERS so we can take all the blame :)

Signed-off-by: Gao Xiang <gaoxiang25@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>